The present invention relates generally to a diverter apparatus and methods and more particularly to a drill string diverter apparatus which will redirect fluids that have entered a casing string while the casing string is run into a wellbore.
In the construction of oil and gas wells, a wellbore is drilled into one or more subterranean formations or zones containing oil and/or gas to be produced. The wellbore is typically drilled utilizing a drilling rig which has a rotary table on its floor to rotate a pipe string during drilling and other operations. During a wellbore drilling operation, drilling fluid (also called drilling mud) is circulated through a wellbore by pumping it down through the drill string, through a drill bit connected thereto and upwardly back to the surface through the annulus between the wellbore wall and the drill string. The circulation of the drilling fluid functions to lubricate the drill bit, remove cuttings from the wellbore as they are produced and exert hydrostatic pressure on the pressurized fluid containing formations penetrated by the wellbore to prevent blowouts.
In most instances, after the wellbore is drilled, the drill string is removed and a casing string is run into the wellbore while maintaining sufficient drilling fluid in the wellbore to prevent blowouts. The term "casing string" is used herein to mean any string of pipe which is lowered into and cemented in a wellbore including but not limited to surface casing, liners and the like. As is known in the art, the term "liner" simply refers to a casing string having a smaller outer diameter than the inner diameter of a casing that has already been cemented into a portion of a wellbore.
During casing running operations, the casing string must be kept filled with fluid to prevent excessive fluid pressure differentials across the casing string and to prevent blowouts. Heretofore, fluid has been added to the casing string at the surface after each additional casing joint is threadedly connected to the string and the casing string is lowered into the wellbore. Well casing fill apparatus have also been utilized at or near the bottom end of the casing string to allow well fluid in the wellbore to enter the interior of the casing string while it is being run.
One purpose for allowing wellbore fluid to enter the casing string at the end thereof is to reduce the surge pressure on the formation created when the casing string is run into the wellbore. Surge pressure refers to the pressure applied to the formation when the casing being run into the wellbore forces wellbore fluid downward in the wellbore and outward into the subterranean formation. One particularly useful casing fill apparatus is disclosed in U.S. Pat. No. 5,641,021 to Murray et al., assigned to the assignee of the present invention, the details of which are incorporated herein by reference. Although such casing fill apparatus work well to reduce surge pressure, there are situations where surge pressure is still a problem.
Liners having an outer diameter slightly smaller than the inner diameter of casing that has previously been cemented in the wellbore are typically lowered into a partially cased wellbore and cemented in the uncased portion of a wellbore. The liner is lowered into the wellbore so that it extends below the bottom end of the casing into the uncased portion of the wellbore. Once a desired length of liner has been made up, it is typically lowered into the wellbore utilizing a drill string that is connected to the liner with a liner running tool. The liner will typically include a well casing fill apparatus so that as the liner is lowered into the wellbore, wellbore fluids are allowed to enter the liner at or near the bottom end thereof.
Because the drill string has a much smaller inner diameter than the liner, the formation may experience surge pressure as the fluid in the liner is forced to pass through the transition from the liner to the drill string and up the smaller diameter drill string. Thus, there is a continuing need for an apparatus that will reduce the surge pressure on the formation when lowering a liner into a wellbore.